FM Deviation Measurements
1. Bessel Zero Method
The method makes use of the fact that the carrier amplitude of an FM transmission is zero at certain precise modulation indices. The modulation index is defined as the peak deviation divided by the modulation frequency and the values in the table are derived from a Bessel function.
In order to use this method the carrier of the FM transmission must be monitored using a separate, narrow band SSB/CW receiver, preferably with a narrow band filter. Set the modulation frequency to 1KHz and zero deviation. Tune the receiver to the transmitter frequency so that the carrier beat note is clearly audible. Now slowly increase the deviation and you will hear the carrier decrease to zero and then reappear - when the carrier is inaudible the deviation will be 2.405KHz. Continue increasing the deviation and the next carrier null will occur at 5.52KHz deviation. The nulls are very sharp so they are easy to miss.
The following table gives the sequence number and value of the modulation index for zero carrier amplitude (null) commencing from zero deviation.In order to obtain the correct deviation we calculate the required audio frequency using the formula:
Audio Frequency = Deviation / Modulation Index.
Null Number
Modulation Index
Null Number
Null Number
1
2.405
6
18.071
2
5.520
7
21.212
3
8.654
8
24.353
4
11.792
9
27.494
5
14.931
10
30.635
For 2.5KHz deviation look for the first null with a modulation frequency of 1039Hz and for 5KHz deviation look for the second null with a modulation frequency of 906Hz.
2. Modulation Meter
The following block diagram shows how to make a simple, manually tuned modulation meter. The function is that of a single superhet with both amplitude and frequency modulation detectors and an AF amplifier driving a detector and meter.
The local oscillator may be 100KHz above or below the incoming frequency. The IF is very low in frequency so the signal will appear at two closely spaced points in the tuning range. The IF is also fairly broad so tuning is not too critical - selectivity is determined by simple filtering immediately after the mixer.
The IF feed to the AM detector must be taken before the limiter to avoid any distortion.
The FM detector uses a pulse counting discriminator which must be driven by a rectangular waveform with a fast rise and fall time. A Schmidt trigger is normally used here, driven in turn by the limiting IF amplifier. A simple tuning indicator is required so a DC output from the AM detector can be used with a "Tune" position on the mode switch.
More information to follow.
